cmder `clink.lua` to show number of commits to push. See comment below for description.

clink.lua

function get_git_num_unpushed_commits(branch_name)
    for line in io.popen("git rev-list --count origin/"..branch_name..".."..branch_name.." 2>nul"):lines() do
        return tonumber((string.gsub(line, "%s+", "")))
    end
    return 0
end

function git_prompt_filter()

    -- Colors for git status
    local colors = {
        clean = "\x1b[1;37;40m",
        dirty = "\x1b[31;1m",
        unpushed_commits = "\x1b[38;5;123m",
    }

    local git_dir = get_git_dir()
    if git_dir then
        -- if we're inside of git repo then try to detect current branch
        local branch = get_git_branch(git_dir)
        if branch then
            -- Has branch => therefore it is a git folder, now figure out status
            if get_git_status() then
                color = colors.clean
            else
                color = colors.dirty
            end

            local unpushed_commits = get_git_num_unpushed_commits(branch)
            if (unpushed_commits > 0) then
                branch = branch..colors.unpushed_commits.." ➚"..unpushed_commits..color
            end

            clink.prompt.value = string.gsub(clink.prompt.value, "{git}", color.."("..branch..")")
            return false
        end
    end

    -- No git present or not in git file
    clink.prompt.value = string.gsub(clink.prompt.value, "{git}", "")
    return false
end

Adds number of commits that the branch is ahead of origin, something like:

There is an assumption that the remote is origin. Also, this is my first attempt at Lua, some things are probably wrong.